Quaternary International 409 (2016) 44e49 Contents lists available at ScienceDirect Quaternary International journal homepage: www.elsevier.com/locate/quaint Chocolate clays of the Northern Caspian Sea Region: Distribution, structure, and origin * R.R. Makshaev , A.A. Svitoch Moscow State University, Department of Geography, Moscow, Russia article info abstract Article history: The chocolate clays commonly comprise a part of the Lower Khvalynian sequence in the northern Available online 29 July 2015 Caspian Lowland and in the Volga region. The mode of occurrence varies from continuous to patchy mosaic. All the clay deposits are confined to Pre-Khvalynian depressions of various origins. There are Keywords: several sub-facies distinguishable within the chocolate clay facies: mono-clayey (typologic), stratified Chocolate clays sandy-clayey and silty-clayey. Judging from specific features of lithology, geomorphic position, mode of Facies occurrence, mollusc fauna composition, and radiocarbon dates, the chocolate clays represent a specific Northern Caspian lowland facies of the Lower Khvalynian sediments and cannot be considered as an individual stratigraphic unit. Mode of occurrence © Composition 2015 Elsevier Ltd and INQUA. All rights reserved. Lower Khvalynian 1. Introduction spatial distribution, occurrence and structure, characteristics of the clay facies and lithology, color, and the type of fossil mollusc fauna. Among the Pleistocene marine deposits of the Caspian Sea, one of the most interesting facies are the “chocolate clays”. They are 2. Materials and methods of the analysis widely distributed in the Volga and Ural river valleys of the North Caspian Sea region in the form of sporadic or continuous covers. All materials and data about chocolate clays have been obtained The first description was obtained by Baer (1856) from outcrops by field research expeditions in key sections of the North Caspian near Enotaevka in the Volga River valley. Detailed description of a Sea Region and the Volga Region. A wide range of methods was section that contained chocolate clays was first introduced by P.A. used, such as lithological, mineralogical, geomorphological, mala- Pravoslavlev. The first researchers described chocolate clays as cofaunistic and radiocarbon. The key sections were minutely layer- chocolate-brown slaty clays. A laconic and concise description of by layer studied, fossils recovered from the sections were the chocolate clays was proposed by Shantser (1951): “… Chocolate described, and 14C dates were obtained. clays represent horizontal bedding that contains alternation of thin dense layers, heavy clays and light silty clay”. These layers, espe- 3. Results cially clays, split into parallel platy parting. Since K. Baer first mentioned the clays in 1856, they were dis- 3.1. Spatial distribution of the chocolate clays cussed in a number of papers (Pravoslavlev, 1908; Britsyna, 1954; Priklonsky et al., 1956; Fedorov, 1957; Arkhipov, 1958; Vasiliev, The chocolate clays are mostly confined to the middle and lower 1961; Obedientova, Gubonina, 1962; Svitoch, Yanina, 1997; reaches of the Volga R. (Fig. 1), including the left and right sides of Badyukova, 2000; Chistyakova, 2001; Shantser, 1951; Leonov the Volga and Akhtuba valleys and the Volga R. delta, as well as the et al., 2002; Tudryn et al., 2013). In spite of long-term in- Kalmykia region adjoining the Volga valley. They are found also vestigations performed by many specialists, numerous problems within a linear zone of depressions from the Kaisatskoe settlement are still under discussion, including the topic considered below: are to Elton and Verkhniy Baskunchak lakes, and in the Ural R. valley. the chocolate clays to be considered as a specific formation or The largest area of the chocolate clays is found in the Volga valley, facies? To analyze the problem, the authors give attention to the on its 2nd terrace between the city of Samara and the Yenotaevka settlement. Farther east, on the Volga-Ural interfluve, typical chocolate clays practically disappear (Svitoch, 1968). The clays * Corresponding author. occur in the Ural R. valley and form the sedimentary cover of the E-mail address: [email protected] (R.R. Makshaev). Khvalynian plain. http://dx.doi.org/10.1016/j.quaint.2015.07.018 1040-6182/© 2015 Elsevier Ltd and INQUA. All rights reserved. R.R. Makshaev, A.A. Svitoch / Quaternary International 409 (2016) 44e49 45 Fig. 1. Spatial distribution of chocolate clays in the Northern Caspian and Volga regions. (I e Middle reach of the Volga R, II e left side of the Lower Volga valley, III e right side of the Lower Volga valley, IV e delta Volga, V e Kalmykia, VI e Kaysatskoye e Elton e Verkhnyi Baskunchak, VII e Ural River valley). In the middle reach of the Volga R., the chocolate clays were Caspian Lowland. It was here, at the Chorny Yar (~À5 m a.s.l., N studied by Mazarovich (1935), Koptev (1966), and Obedientova and 4801055.4700,E4606043.0400) section, that they were first described Gubonina (1962). The clays form a continuous mantle over the 2nd by Baer (1856). Later, they attracted an interest of many specialists terrace of the Volga R. and of its large tributaries, such as Bolshoi (Pravoslavlev, 1929; Nikolaev, 1935; Zhukov, 1945; Kovda, 1950; Irgyz, Maly Irgyz, Yeruslan, and Bolshoi Cheremshan. They penetrate Britsyna, 1954; Fedorov, 1957; Arkhipov, 1958; Vasiliev, 1961; far into the interfluvial plateau along ancient erosional landforms. Menabde et al., 1991; Svitoch et al., 1995; Svitoch and Yanina, 1997). They are found in the Bolshoi Irgyz valley 70 km east of the Volga R. The deposits are exposed in picturesque erosional scarps on the The zone of their occurrence in the ancient Volga estuary between left bank of the Akhtuba (the left distributary of the Volga R.) and the Caspian Lowland and the Zhiguli Heights is 2 to 30e40 km wide on the right side of the Volga. The latter displays the section of the (Obedientova and Gubonina, 1962) and about 600 km long (from S ancient Volga estuary filled with the chocolate clays and other to N). The northernmost point of occurrence of the chocolate clays is marine deposits. The top of the clays is at 25e20 m a.s.l. in the north recorded at the Bolshoi Cheremshan River mouth. The chocolate of the Caspian Lowland and descends gradually southward to À5m clay interlayers are found in the sequence of the 1st terrace of the at the Chorny Yar and Selitryannoye settlements and to 10 m below Volga within the reservoir area of the Volzhskaya hydro-electric sea level at the head of the Volga delta. The depth of the Pre- power station (Obedientova and Gubonina, 1962). Over the greater Khvalynian erosion (which controls the thickness of the chocolate part of the Volga valley in its middle reaches the top of the chocolate clays) in depressions averages to 2e4 m, although it may change clays occurs at an altitude from 30e40 m to 25e20 m a.s.l. gradually over a small distance. For example, the clays are 10 m thick in the lowering towards the Caspian Lowland (Vasiliev, 1961; Moskvitin, Svetly Yar (~5 m a.s.l., N 4829010.3700,E4446036.4200) section, 1962; Obedientova and Gubonina, 1962). while 5 km south, at Raygorod (~6 m a.s.l., N 4825049.8200,E The Khvalynian deposits in the middle reaches of the Volga are 4457041.1000), the thickness is reduced to 2 m, which suggests a dominated by the chocolate clays that occur usually at the base of the considerable irregularity of the Pre-Khvalynian bed topography. sequence. In the north of the region (Chapaevsk (~28 m a.s.l., N On the left side of the Lower Volga valley and its distributary 5253041.3700,E4938004.4300), Maly Karaman (~30 m a.s.l., N Akhtuba, the chocolate clays are exposed in the sections of Bykovo, 5140018.2000,E4650013.5400) and Rovnoye (~15 m a.s.l., N Osadnyaya, Srednya Akhtuba, and Selitrennoe. Their position and 5045027.9000,E4601041.3800) sections), the clays are overlain by structure vary considerably from one exposure to another. At the stratified sands and loams. Farther south (Novoprivolnoye northern localities of Bykovo and Srednaya Akhtuba (~15 m a.s.l., N (~18 m a.s.l., N 5048008.0300,E4605022.0500) and Torgun (~15 m a.s.l., 4841054.2200,E4454033.2600), the sequence is dominated by the N5017033.0400,E4557055.2800) sections), the Khvalynian sequence chocolate clays underlain or overlain with sands. In the middle part, is more complicated, including several members of the chocolate interbeds of sand and silt with shells of index Khvalynian mollusks clays in the upper part and at the base of the section. Typically, the are common. In the south, the chocolate clays also include sand and index species of the brackish-water mollusc fauna are completely silt interbeds with shells of index species Didacna protracta, mostly absent, while freshwater molluscs are found occasionally. in the central part of the sections. Noticeable variations in the The Lower Volga region is known as the largest area of the sedimentation environments occur from south to north. chocolate clay distribution. They form the vast 2nd terrace of the On the right side of the Lower Volga valley, chocolate clay out- Volga and occur in depressions of the Early Khvalynian plain on the crops are found in abundance from Volgograd to Lenino settlement. Download English Version: https://daneshyari.com/en/article/1039985 Download Persian Version: https://daneshyari.com/article/1039985 Daneshyari.com.